The present invention relates to a manufacturing method for a micromechanical device, in particular for a micromechanical vibrating mirror.
Although applicable to any micromechanical devices, the present invention and the objective underlying it are clarified with reference to a micromechanical vibrating mirror.
Several design variations exist for micromechanical vibrating mirrors. For example, an aluminum membrane can be deflected by electrostatic forces, as is described in Texas Instruments, L. J. Hornbeck, Proc. Soc. Photo-Opt. Instrum. Eng. 1150 (1989) 86; J. Bxc3xchler et al. J. MEMS 6 (1997) 126.
Several publications describe movable mirrors made of monocrystalline silicon (IBM, K. E. Petersen, IBM J. Res. Develop. 24 (1980) 631) or of polysilicon (CSEM, V. P. Jaecklin et al., Proc. IEEE Micro Electro Mech. System Workshop, FL, USA (1993) 124).
The problem on which the present invention is based is that these concepts, above all in the case of large mirrors having lateral dimensions in the range of a few 100 xcexcm, allow only small angular deflections in the range of a few degrees, which can be attributed to the limited, maximum possible edge deflection. In the case of surface-micromechanical concepts, the edge deflection is limited by the small substrate clearance of a few micrometers, while in the case of bulk-micromechanical components, the thick torsion-spring suspension mounts permit only relatively small torsion angles, even given large drive voltages, or manufacturing tolerances give rise to substantial variations in the mirror properties.
Compared to the known design-approach starting points, the manufacturing method of the present invention, has the advantage that large deflection angles can be achieved without making the process technology more complicated.
According to the present invention a micromechanical device, in particular a resonant vibrating-mirror device made of silicon, is flexibly suspended on connecting webs, preferably long bars made of silicon, in such a way that it can turn about its longitudinal axis. Underneath the resonant vibrating-mirror device, the bulk material is completely removed, i.e., the wafer is locally perforated, so that the device can perform torsional vibrations about the connecting webs, the torsional vibrations having such an amplitude that a part of the device extends into the region of the removed bulk material.
In the case of a resonant vibrating-mirror device, the reflecting surface of the mirror is used simultaneously as an actuator for adjusting the mirror. A counter-electrode, placed under the mirror by aligned counter-bonding, for example, forms a capacitor with the mirror surface. The mirror element is deflected by applying an electric voltage. The device can be operated at ambient pressure, since because of the great distance between the mirror and counter-bonded driving electrodes, there is only a relatively small air friction. If the mechanical resonance of the mirror element is excited by the actuators, the deflection of the actuator attainable due to the electrostatic forces is enhanced by the factor of the mechanical quality.
According to a another embodiment of the present invention, first the region of the third layer is etched through, then the first layer is etched through, and thereupon the region of the second layer is removed.
According to another embodiment of the present invention, the region of the third layer is etched through by anisotropic etching of the back surface. Deep patterns can be etched into silicon inexpensively using established wet etch techniques such as KOH, TMAH. The necessity of compensation structures for protecting convex corners has an effect on the patterning.
According to a another embodiment of the present invention, the first layer is etched through by a dry etching process. Only micromechanical spring suspensions having lateral dimensions of  greater than 100 xcexcm can be implemented in a controlled manner using wet etch techniques. In contrast, in surface micromechanics, any patterns as desired having lateral dimensions of 10 xcexcm and less with substantial aspect ratios can be etched vertically into the substrate by dry etching processes (plasma trenching). However, deep etching  greater than 100 xcexcm is not actually practical for reasons of cost.
The method of the present invention for manufacturing a micro-mirror with large deflection angles is preferably composed of a combination of laterally high-resolution dry etching of the mirror structure on one hand, and the customary deep etching of the back-surface structure on the other hand.
According to another embodiment of the present invention, first of all the first layer is etched through, and after that the region of the second layer is removed and the region of the third layer is etched through.
Yet another embodiment of the present invention provides for etching through the region of the third layer by anodic etching of the back surface to make the region of the third layer porous, and subsequently removing the region which has been made porous. In this case, no protection (protective layer or etching dosage) is necessary on the front side; rather, only a mask made, for example, of gold or chromium/gold or SiO2+Cr+Ai, etc., is necessary on the back surface. In the case of large elements, the surface consumption for (100)-standard-wafers is perceptively reduced compared to KOH-etched backside cavities.
In another embodiment of the present invention, the region of the second layer is directly removed in the anodizing electrolyte. No additional etching of the middle second layer, e.g., sacrificial oxide etching, is necessary, because the second layer, e.g., the oxide, is etched by the anodizing electrolyte. The anodization is not critical in semi-conductor production; no KOH and no NaOH, etc., are used, but rather CMOS-compatible process liquids such as hydrofluoric acid and DI water a deionized water.
According to another embodiment of the present invention, the counter-electrodes are applied on a base, and the base is bonded to the third layer in such a way that the counter-electrodes lie essentially opposite the island region. In this context, the counter-electrodes for the capacitive driving of the mirror are expediently counter-bonded on suitable insulating material.
In another embodiment of the present invention, an SOI (i.e., silicon-on-insulator) structure having an SOI layer, which is provided on a silicon substrate layer with the interposition of an insulating layer, is supplied as the three-layer structure. This is a customary standard structure in micromechanics.
Another embodiment of the present invention provides for using narrow regions of the first layer as connecting webs which are formed by the etching process using a suitable mask geometry. In the case of the vibrating-mirror device, they can be used as torsion springs.
According to still another embodiment of the present invention, narrow regions of a preferably metallic additional layer are used as connecting webs. In the case of the vibrating-mirror device, these can be used as torsion springs. In the implementation of other devices, the connecting webs can also be used as bearing elements.